1. Field of the Invention
The present invention relates to a hemispherical bearing apparatus. In particular, the hemispherical bearing apparatus has a coating of molybdenum disulfide (MoS.sub.2) on either the surface of a hemispherical recessed portion formed in a bushing or the hemispherical surface of a hemispherical member opposed to the hemispherical recessed portion.
2. Description of the Related Art
Recently, with rapid developments in the computer industry, a variety of driving motors for driving various apparatuses such as a polygon mirror driving apparatus of a laser printer, a spindle motor of a hard disk and a VCR head driving motor, require highly accurate and super high speed rotation of a rotating member thereof. The rotating member generally has a rotary shaft at the rotational center. The driving motor must achieve high speed rotation without vibration or oscillation of the rotary shaft of the rotating member to perform data retrieving, saving and regenerating operations more quickly and with more accuracy.
For the rotating member to achieve high accuracy and super high speed rotation without vibration or oscillation, various motors together with various hydrodynamic bearing apparatuses for rotating the motors have been developed. In the various hydrodynamic bearing apparatuses, hemispherical bearing apparatuses have been developed that support the radial load and the thrust load at the same time and are suitable for super high speed rotation.
A conventional polygon mirror driving apparatus of a laser printer including such a hemispherical bearing apparatus will be described with reference to FIG. 1.
The polygon mirror driving apparatus includes: a fixing shaft 20 that is a rotational center of a polygon mirror 10; hemispherical members 30 and 35 having a hemispherical surface with a high sphericity and press-fitted onto the fixing shaft 20; a bushing 40 for supporting a radial load and a thrust load of the hemispherical members 30 and 35; a rotor 55 and a stator 50 that are driving apparatuses; a hub 60; and a lower housing 70.
The connection of the polygon mirror 10, the fixing shaft 20, the hemispherical members 30 and 35, and the bushing 40 will be described hereinafter. The fixing shaft 20 is press-fitted into the lower housing 70. The hemispherical members 30 and 35 are press-fitted onto the fixing shaft 20. A dynamic pressure generating groove (not shown) for generating a predetermined hydrodynamic pressure is formed on the hemispherical surfaces of the hemispherical members 30 and 35. The hub 60 on which the polygon mirror 10 and the rotor 55 are installed is press-fitted onto the bushing 40. The hemispherical members 30 and 35 and the fixing shaft 20 are fixed and the bushing 40 can be rotated with respect to the fixing shaft 20.
The bushing 40 supporting the radial load and the thrust load of the hemispherical members 30 and 35, has a cylindrical shape and includes therein a through hole having a larger diameter than the diameter of the fixing shaft 20. The bushing 40 also includes recessed portions 30a and 35a that are symmetrically formed in both ends thereof. The sphericities of the recessed portions 30a and 35a are the same as the sphericities of the hemispherical members 30 and 35, respectively. A spacer 40a is inserted into the through hole of the bushing 40 between the hemispherical members 30 and 35 in order to form clearances between the hemispherical members 30 and 35 and the hemispherical recessed portions 30a and 35a.
Referring to FIG. 2A, the surface of the hemispherical recessed portion 35a formed in the bushing 40 is coated with a titanium nitrogen (TiN) layer 32. Referring to FIG. 2B, the surface of the hemispherical member 35 opposed to the hemispherical recessed portion 35a is coated with a double layer of a TiN layer 32 and a diamond-like carbon (DLC) layer 34 on the TiN layer 32. The TiN layer 32 has a high resistance against abrasion and can increase the durability of the recessed portion 35a and the hemispherical member 35. The DLC layer 34 has lower friction factors and can increase the lubricity of the hemispherical member 35.
However, the bearing apparatus including such a bushing and hemispherical members suffers several disadvantages.
First, costly coating equipment and a great amount of time are required for coating the bushing and the hemispherical members with the TiN layer and the DLC layer. Typically two hours is taken for forming the TiN layer or the DLC layer of one micron thick. Accordingly a great amount of time is taken for forming the double layer of several microns.
Second, since the coating process is carried out at a high temperature of 500.degree. C., the material for forming the bushing and the hemispherical members is limited accordingly. For example, if the hemispherical member is formed of aluminum, that melts at 300.degree. C., the bushing and the hemispherical members cannot be coated with the TiN layer or the DLC layer at the temperature of 500.degree. C. Accordingly, only iron material can be used as the material for the bushing and the hemispherical members.